1. Field of the Invention
The present invention relates to a simplified and more integrated approach for fabricating solar energy panels for the large scale generation of electric power by a fully automated procedure. More particularly, the present invention relates to an automated method of fabricating solar panels whose conversion efficiency is at least as great as any panels now assembled by hand.
2. Description of the Prior Art
Solar panels have been prepared in the past by a variety of techniques and are generally characterized by containing a multiplicity of solar cells supported on a suitable support material. Each cell is provided with electrodes for the transport of current generated by incident photons, and is conductively interconnected with the other cells of the panel so that all of the current generated by the individual cells is collected and available to perform work. U.S. Pat. No. 3,780,424 shows a silicon cell solar array in which a series of solar cells are provided with grid systems which in turn are connected to bus bars for the collection and distribution of electric current. The solar panels are formed by supporting the cells on a polyimide substrate via a layer of adhesive material such as fluorinated ethylene-propylene copolymer. A protective cover layer of fluorinated ethylene propylene copolymer is also provided. U.S. Pat. No. 3,658,596 shows a solar cell modular assembly in which silicon photovoltaic cells are fused between two sheets of fluorinated ethylene propylene copolymer wherein the solar cells are provided with negative grids and collectors. U.S. Pat. No. 3,849,880 discloses a method of fabricating a solar cell array in which individual solar cells are positioned on preprinted areas of a substrate by an adhesive. A region of the bottom of each cell is not coated with adhesive in order to leave portions thereof available for the attachment of interconnectors. The complicated procedure further requires that the substrate must be prepunched so that apertures are available at positions of contact of an interconnector and the bottom of each cell. Electrical interconnectors must then be slid into position such that they touch the top electrode of at least one cell and the base electrode of an adjacent cell. The interconnectors are then welded directly to the top electrodes and through the prepunched apertures in the substrate to the base electrodes. These prior art techniques have the common disadvantage that they are too costly and complicated to permit the facile commercial production of solar panels for use in industrial, commercial and residential power applications. Moreover, none of the prior art techniques of fabricating solar panels are fully automated, which is a factor which increases the cost of production of solar units.
The individual solar cells which are commonly formed are heterojunction and Schottky barrier devices. Examples of heterojunction devices include Ga.sub.1-x Al.sub.x As-GaAs, ZnSe-GaAs, GaP-Si and ZiS-Si devices. Solar cells of the Ga.sub.1-x Al.sub.x AS-nGaAs type are commonly fabricated by liquid phase epitaxy, while nZnSe-pGaAs devices have been fabricated by both liquid phase epitaxy and vapor growth. While the LPE formed ZnSe-GaAs device exhibits low conversion efficiency, the device exhibits good spectral response characteristics at high photon energies which verifies the theory that the recombination velocity should be low at the heterojunction. Epitaxial ZnS-Si and GaP-Si heterojunction devices can be fabricated by a number of methods. However, in the preparation of these devices the Si substrate must be cleaned of surface oxide. Moreover, at the temperature of fabrication of the GaP-Si devices, a major problem encountered is the cracking of the GaP layers created by the stress caused by the thermal expansion difference between GaP and Si.
Schottky barrier devices are the simplest types of solar cells to prepare in that they only require an ohmic contact on the back of the devices and a transparent metal at the front. The transparent metal film is usually deposited by evaporation and is usually of an expensive noble metal such as platinum, gold, silver or the like. Another type of heterojunction or Schottky device which is receiving attention is a device which has a transparent conductive glass of a material such as In.sub.2 O.sub.3, SnO.sub.x, ZnO or the like. The oxide layer is highly transparent, usually above 90% for layers several thousand angstroms thick. Several devices on this order have been made using SnO.sub.2 on Si and GaAs substrates but their efficiencies have been low, probably caused by the poor quality of the SnO.sub.2 films. However, Anderson, Glass-Si Heterojunction Solar Cells, NSF RANN Report: HER 74-17631, July, 1975 has reported In.sub.2 O.sub.3 and SnO.sub.2 coated silicon semiconductor devices wherein the SnO.sub.2 coated device achieved an efficiency of 10%. SnO.sub.2 coated devices are commonly prepared by evaporating or sputtering Sn onto a substrate and then oxidizing with oxygen. Alternatively, an SnCl.sub.4 film can be oxidized. Dubow et al. (Colorado State University) have reported In.sub.2 O.sub.3 silicon solar devices which have conversion efficiencies of 3.3%.
The difficulty with the prior art solar cell devices is that the solar panel arrays are formed by fabricating and placing individual cells within a panel and then coupling all of the cells into conductive relationship by soldering or welding all interconnection wires or metal strips followed by some form of protective covering. Thus, the prior art procedures are multi-step techniques, and because of the relative complexity of the preparative procedures, the resulting panels are relatively expensive. A need, therefore, continues to exit for a fully automated method of producing solar cell panels which would be industrially advantageous, thereby leading to decreased costs of production.